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Y. J. CHOI¹, K. L. McCarthy, and M. J. McCarthy. (1) Biological and Agricultural Engineering, University of California, Davis, One Shields Ave., Davis, CA 95616-8598

Mixing in a twin screw extruder, as with other mixing equipment, is a process that reduces composition nonuniformity. This study used a noninvasive technique to quantify mixing as a function of axial position in an APV 30 mm corotating twin screw extruder. Two fluids of dissimilar NMR properties were mixed in a pilot-scale twin screw extruder. The extent of mixing was evaluated statistically at axial cross sections of concentration images obtained experimentally by magnetic resonance imaging (MRI). The extruder (L/D of 13.5) was fabricated with transparent acrylic barrel and fiberglass-filled double lead nylon screws. It was loaded with two viscosity-matched test fluids, selected for their difference in NMR relaxation times: fluid 1 with T1 of 2000 ms (T2 of 430 ms, density of 1.015 g/mL) and fluid 2 with T1 of 253 ms (T2 of 56 ms, density of 1.027 g/mL). A multislice imaging sequence was performed to obtain concentration images over 10 cm sections of the extruder. Each slice was 15 mm thick with 160 mm field-of-view. Concentration profiles were obtained by MRI to provide information regarding mixing in consecutive axial slices. At the extruder inlet, histograms of signal intensity identified the two fluids as a bimodal distribution prior to mixing. As mixing proceeded, the bimodal distribution became unimodal at an intermediate signal intensity. Quantitatively, the extent of mixing was characterized best in terms of coefficient of variation, which allows comparison from one image to another. In these studies, the coefficient of variation showed a 25% decrease over a distance of 30 mm. The MRI technique will be used to differentiate between axial mixing (typically measured in classical RTD studies) and transverse mixing that takes place in the extruder channel.